The present invention relates to a control system for an electromagnetic clutch for an automatic transmission of a motor vehicle.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A No. 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes such as a starting mode of a vehicle, reverse excitation mode, drag mode, and mode of lock-up engagement. One of the modes is selected in accordance with a position of a selector lever and driving conditions to control the electromagnetic clutch.
In the drag mode, a small clutch current flows in a coil of the electromagnetic clutch when an accelerator pedal is released in a low vehicle speed range thereby applying a small drag torque to a driven member of the clutch to reduce clearances formed in the transmission. Accordingly, at a start of the vehicle or reacceleration of the vehicle at low vehicle speed, the starting torque necessary for driving the driven member is reduced, and hence, the power of an engine is properly transmitted to driving wheels through the transmission, so that the vehicle can be smoothly started or reaccelerated.
However, in the continuously variable belt-drive transmission, the starting torque needed for the reacceleration increases as the vehicle speed decreases. In the system of the afore-mentioned prior art, the drag torque is constant regardless of the vehicle speed. Accordingly, the drag torque is not effectively exerted on the transmission at the reacceleration.